Modem snowmobiles typically have two skis for supporting the front of the machine and to provide steering and a rubber track for supporting the rear of the machine and to provide traction force. The rider is typically positioned anywhere from directly over the center of the track to over the front of the track, while the engine is located between the drive track and the skis, usually as low as possible.
A snowmobile is considered to be an off-road vehicle and as such it must be able to contend with many different riding environments. The terrain may change from smooth trails to large bumps and jumps. Snow conditions can vary from grippy hard-pack to bottomless powder, from smooth trails to rough and bumpy ones. Accordingly, modem snowmobiles are equipped with long travel suspensions at the front (skis) as well as at the rear (track). The rear track suspension is mounted to the underside of the snowmobile within a tunnel that partially encloses the track and suspension. A seat for the snowmobile rider is provided on top of the tunnel and running boards are provided on either side for supporting the rider's feet.
Existing snowmobile rear suspensions are quite similar. A rubber track is driven from the front by a set of cogged drivers. A set of slide rails enclosed within the track pushes the track onto the ground and provides a sliding surface for the track and mounting points for wheels and suspension arms. There are generally two sets of pivoting suspension arms connecting the rails to the tunnel, one positioned in front of the other at similar angles to form a parallelogram. In most suspensions, both arms are biased downwards with two separate springs. The springs can be arranged in a number of ways to provide various motion ratios (i.e., spring compression versus suspension movement). Each spring is typically controlled by a hydraulic shock absorber to allow the suspension to release bump energy slower than it is absorbed and to prevent un-dampened oscillation.
A snowmobile's rear suspension must be able to perform the following functions under all conditions: isolate the rider and the machine from the full bump force; maintain track contact with the ground by allowing the rails to articulate to some degree; and, allow the snowmobile to “lean back” upon acceleration to transfer weight to the track.
In most current designs, bump absorption is obtained by using springs and shocks which are mounted in various ways to provide the desired motion ratio. A motion ratio compares spring and shock compression with suspension compression. A falling rate motion ratio is one where the shock speed decreases as the suspension compresses at a constant rate. This results in a soft and comfortable ride, but has poor resistance to bottoming over large bumps and upon landing after being airborne. A rising rate motion ratio is one in which the shock speed increases as the suspension compresses at a constant rate. This setup is typically found in sport snowmobiles and provides excellent control at high speeds and excellent bottoming protection.
Because the off-road environment is often unsmooth, the slide rails must be able to pitch forward and backward to some degree in order to maintain full track contact with the ground. If the arms and rails formed a solid parallelogram, this would not be possible. Instead, this function is typically accomplished by using a solid mounting point for only one of the arms (usually the front arm). The other (back) arm mounts indirectly to the rails using either a third smaller arm or some sort of telescoping device. The amount of rail pitch must be limited to eliminate excessive pitching of the entire machine. This is done in several different ways, usually by limiting the rotation of the third arm or limiting the amount of axial movement of the telescoping device. Once the limit is reached, the suspension is coupled, and the suspension geometry approximates a parallelogram. This forces both arms to compress at the same time. The moment at which a limit is reached is sometimes referred to as the “coupling moment”. Most suspensions are coupled in both directions (front to back and back to front), but a few are coupled only in one direction, allowing unrestricted movement of the front arm or of the rear arm.
A snowmobile suspension must also provide weight transfer. Because snow provides limited traction, it is important to put as much of the vehicle weight over the track as possible during acceleration. This is typically accomplished by using the track tension during acceleration to actively pitch the rails, lifting the front of the machine to some degree. Upon acceleration, the top part of the track is in tension. A force analysis performed at the rear axle of the suspension typically shows a major component pushing the rails forward and a minor component pulling the back of the rails upward. The major forward push on the rails is transferred to the front arm where, because of its angle, it pushes the centre of the machine upwards, increasing the downward force on the front of the rails. The minor component pulling up on the back of the rails pulls the rear of the machine downwards, decreasing the downward force on the back of the rails. The result is a snowmobile with less weight on its skis and more on its track.
As mentioned above, in a typical snowmobile riding environment, bumps of all sizes may be encountered. A certain amount of rail pitch is beneficial to allow the track to maintain better contact over the smaller bumps. In some situations, however, the amount of rail pitch must be limited. This is accomplished by several means, usually involving a stopper or a bumper of some sort. The movement of a shorter third arm or a telescoping device is limited by a rubber bumper or a similar device.
When the track suspension encounters a large bump at high speed, first the front arm compresses relative to the underside of the snowmobile. Without any coupling device, the front spring may be overcome by the large bump force and the front arm may bottom harshly. If the two arms become coupled at a certain point, both front and rear arms are forced to move together. This allows the bump to be absorbed by both front and rear springs, effectively increasing the amount of bump energy that can be absorbed using a given set of springs. It is also possible to simply use stronger springs on the front arm, but this results in a harsher ride.
Another scenario in which coupling is desirable is in tail first landings. It is not uncommon for snowmobilers to launch the entire machine into the air, often landing “tail first”. In this case, the rear arm in the suspension is prone to rapid bottoming. In this case, coupling also allows the impact to be absorbed by both suspension arms and their respective springs and shocks.
In most suspensions, coupling is provided by a set of bumpers or a by a set of rods that are able to telescope to a certain extent. At the instant when these suspensions couple, the overall spring force is instantly doubled as both suspension arms are forced to move and both springs are engaged. This is characterized by a harsh ride over “chatter bumps” (i.e., small to medium size evenly spaced bumps that force the coupling device to reach both its limits in rapid succession repeatedly). This is far less desirable than gradual coupling and a smooth increase of overall spring force as the suspension compresses.
There are several currently available rear suspension designs for snowmobiles.
U.S. patent application Ser. No. 10/698,980, filed Oct. 31, 2003 by Imamura et al. and published Aug. 19, 2004, includes a quadrilateral linkage system formed between a vehicle body frame, a front torque arm assembly, a rear torque arm assembly, and an extendable member. This suspension uses one coil-over spring to bias both suspension arms. The shock mounts to each arm a certain distance from its pivot so that as the suspension compresses, the shock also compresses. It is a falling rate design with a multi-rate spring that becomes progressively stiffer as it is compressed. The rear arm is mounted to the rails using a short, vertical arm. The device which limits the amount of rail pitch is the lower extendable member linking both arms together. The extendable member comprises a telescoping rod with adjustable limits.
U.S. Pat. No. 6,390,219 filed May 14, 2001 by Vaisanen discloses a snowmobile suspension that provides a substantially constant motion-ratio (i.e. reduced falling rate) over the entire suspension stroke of the suspension system. The suspension system includes a suspension assembly that includes a lower arm assembly, a suspension arm, and a shock absorber. The lower arm assembly pivotally interconnects the lower portion of the suspension arm and the lower end of the shock absorber to the slide frame at a location relative to the chassis and within the endless track. The upper portion of the suspension arm and the upper end of the shock absorber pivot independently from each other, and the upper portion of the suspension arm is positioned lower and forward of the upper end of the shock absorber. The upper end of the shock absorber is positioned relative to the chassis and within the endless track. The mounting positions defined by (i) the upper end of the suspension arm, (ii) the upper end of the shock absorber, (iii) the lower end of the suspension arm, and (iv) the lower end of the shock absorber cooperate to provide a substantially constant motion-ratio as the slide frame collapses toward the frame element. The rear arm is attached to the rails by an “upside down” third vertical arm.
U.S. Pat. No. 6,234,264 filed Nov. 24, 1998 by Boivin, et al. discloses another snowmobile track suspension. The suspension disclosed by Boivin et al. is a long travel design. The front arm is mounted directly to the tunnel and the front shock is mounted in typical fashion. The front arm is mounted to the rails by means of a sliding pivot in a slot. The rear arm is mounted directly to the tunnel and directly to the rails by means of an adjustable pivot that allows for the necessary rail pitch. The rear shock is mounted in similar fashion to the front one.
While there are numerous rear suspension designs, these designs all have several problems and disadvantages.
One problem relates to slide rail pitching. During acceleration, various suspension forces result in less pressure on the skis. In the foregoing prior art designs, the arrangement of the suspension arms has an unstable geometry. Once the rails begin to pitch (either forward or backward), there is a decreasing amount of resistance to further pitch. The result is that once the rails begin to pitch upon acceleration, they tend to continue to do so until they reach a limit at full transfer at which point the skis are not in contact with the ground. Once the skis begin to lift, they tend to “snap up”, only coming down once the amount of acceleration (throttle opening) decreases by a large amount. While a controllable amount of ski lift is desirable, too much of it results in poor steering and cornering characteristics. Ideally, a rider should be able to control weight transfer more with his body movement than throttle opening.
Another problem exists in designs using a third vertical arm, such as that disclosed by Imamura. As the rail begins to pitch the angle of the third arm changes. A force analysis typically reveals that further pitch becomes progressively easier until a hard limit is reached. This characteristic encourages rapid back and forth pitching within the predetermined limits.
Yet another problem relates to the spring and shock motion ratio. This ratio describes the compression of the shock and/or spring in relation to the upward movement of the suspension rails. As mentioned above, there are three general variations: falling rate, linear, and rising rate. A falling rate suspension has a shock/spring ratio that falls as the suspension is compressed. This causes the suspension to feel softer over large bumps, but can easily bottom out the suspension with large input forces. When a suspension bottoms out, a large “jolt” is fed into the chassis. The result is rider discomfort, potential loss of control, and very high stress levels on suspension components. A rising rate suspension gets more firm as the suspension compresses. This provides excellent bottoming resistance but also results in a firm (bumpy) ride over certain bumps. A constant ratio suspension falls in the middle of the other two, combing some traits of each.
Ideally, a suspension should combine the comfort of a falling rate design with the ability to handle large bumps of a rising rate design. Much effort has been made to this effect. Falling rate suspensions are fitted with multiple springs with different rates in an effort to prevent bottoming. Many shocks are designed so that there is no effective dampening in mid-stroke to allow more comfort over small, rapid stutter bumps. While somewhat effective, these approaches involve compromise between comfort and control.
A need therefore exists for an improved snowmobile rear suspension. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages.